Radar System Engineering

186 THE GATHERING AND PRESENTATION OF RADAR DATA [%c. 6.11
resultant of simultaneously received signals (as is done in the CH by
means of a goniometer; see Sec. 6.9), or by presenting alternately received
signals side-by-side in a type K display and estimating visually their ratio
in height.
The British Type 7 equipment is the most widely used radar exploiting
this signal-comparison principle.
Firstlobe,antenna1
Firstlobe,antenna2 A copy of this equipment built in
. Canada for the United States is
\
\
called the SCR-588; American-built
G / / \ \ redesigns of the same basic equip-
\
g 1/ \ meut are the SCR-527 and the
/ ,
~ \
/’
SCR-627. The antenna of this set.
.= \
In , \ is placed very close to the ground
%’ I \ \
,= ,
in such a way that the beam is
\
.s g ,’
\
\ \ tilted up in elevation and lo\v-angle
/’
coverage deliberately sacrificed. A
\ lobe pattern such as that shown
0123456
in Fig. 6.20 is thus produced.
Elevationangle,n degrees
The lobes are few in number and
FIG.6.20.—Elevationcoveragediagram.
very w-ide. A second antenna,
placed at a different height, produces a second series of lobes which overlap
those of the first antenna. The echo strength for one lobe system is compared
with that for the lobe system from the other antenna. A type K
display system is used which shows the two responses side-by-side on a
12-in. scope. From the relative intensities
and the range of the aircraft,
3
.
height can be deduced.
%
For either lobe pattern, the electric g.gt
~.
field strength at the target depends ~$
w+
I
on the phase difference between the G+
%.1
direct and reflected beams, and so Q.s
~ I
will vary nearly sinusoidally with the
altitude of the target. Figure 6.21
is drawn for antenna heights of 35
ft and 23 ft, on the assumption of
sinusoidal variation. It shows the
om
0 1 2 3 4
Elevationanglein degrees
~lG. 6.21.—Ratio of signalsin lower and
upper beams.
ratio between the signal strength in the lower beam and that in the upper
as a function of elevation angle.
The ratio vanes slowly for small elevation angles, then rapidly as the
lower lobe just touches the aircraft—in this case at an elevation angle of
about 4°. By estimating the signal-strength ratio correct to 10 per cent,
the angle of elevation can be told with an average accuracy of about onethird
degree, corresponding to a height error at 50 miles of + 1000 ft.